1. Field of the Invention
The present invention relates to a variable valve timing apparatus. In particular, the invention relates to a variable valve timing apparatus that varies the timing at which a valve is opened/closed by a variation amount according to an operation amount of an actuator.
2. Description of the Background Art
VVT (Variable Valve Timing) has conventionally been known that changes the phase (crank angle) in (at) which an intake valve or an exhaust valve is opened/closed, according to an operating condition. Generally, the VVT changes the phase by rotating, relative to a sprocket or the like, a camshaft that causes the intake valve or exhaust valve to open/close. The camshaft is rotated by such an actuator as hydraulic or electric motor. Particularly, in the case where the electric motor is used to rotate the camshaft, the torque for rotating the camshaft is difficult to obtain, as compared with the case where the camshaft is hydraulically rotated. Therefore, in the case where the electric motor is used to rotate the camshaft, the torque of the electric motor is transmitted via a link mechanism or the like to the camshaft, thereby rotating the camshaft. However, in the case where the link mechanism or the like is operated to adjust the opening/closing timing, the operation of the actuator is changed in speed (decelerated or accelerated) by the link mechanism or the like to be transmitted to the camshaft. Therefore, in order to accurately control the variable timing, it is desirable that a variation amount by which the opening/closing timing of the valve is varied is proportional to an operation amount or the like of the actuator by which the actuator operates.
Japanese Patent Laying-Open No. 2005-048707 discloses a valve timing adjusting apparatus that adjusts the rotational phase (valve opening/closing timing) of a driven shaft (camshaft) with respect to a driving shaft (crankshaft), in proportion to the rotational phase of a guide member rotated by an actuator. The valve timing adjusting apparatus disclosed in Japanese Patent Laying-Open No. 2005-048707 is provided in a transmission system that transmits a driving torque of the driving shaft (crankshaft) to the driven shaft (camshaft) that drives to open/close at least one of the intake valve and the exhaust valve of an internal combustion engine, and the adjusting apparatus adjusts the opening/closing timing of at least one of the valves. The valve timing adjusting apparatus includes: a phase change mechanism that has a first rotation member rotating in synchronization with the driving shaft and a second rotation member rotating in synchronization with the driven shaft, and that converts motion of a member to be controlled into relative rotating motion of the second rotation member with respect to the first rotation member so as to change the rotational phase of the driven shaft with respect to the driving shaft; and a guide member that is relatively rotated with respect to the first rotation member by transmission of a control torque from the actuator so as to guide a movable body in the direction in which a guide path is extended. The movable body slides in the guide-path-extending direction with respect to the guide member while moving the member to be controlled and accordingly, the rotational phase of the second rotation member with respect to the first rotation member is varied in proportion to the rotational phase of the guide member with respect to the first rotation member.
By means of the valve timing adjusting apparatus disclosed in the above-referenced publication, the movable body relatively slides in the guide-path-extending direction with respect to the guide member while moving the member to be controlled, and thus the rotational phase of the second rotation member with respect to the first rotation member is varied in proportion to the rotational phase of the guide member with respect to the first rotation member. Thus, the rotational phase of the guide member with respect to the first rotation member may be controlled to accurately adjust the rotational phase of the second rotation member with respect to the first rotation member, namely the rotational phase of the driven shaft with respect to the driving shaft.
However, in the case where the variation amount of the opening/closing timing is changed in proportion to the operation amount or the like of the actuator as done by the valve timing adjusting apparatus disclosed in Japanese Patent Laying-Open No. 2005-048707, if the slope of the proportionality is smaller (if the gear ratio of the VVT is higher), a range over which the opening/closing timing can be changed is smaller. On the contrary, in the case where the slope is larger (the gear ratio of the VVT is lower), if the actuator is stopped (in the state where no torque is generated), operation of the engine generates a torque acting on the camshaft and thereby drives the actuator. In this case, the opening/closing timing is changed, and the valve opening/closing timing cannot be kept as controlled.